1. Field of the Invention:
This invention relates generally to telephone communication systems employing bidirectional voice frequency repeaters and more particularly, it relates to an enhanced line powered amplifier, which functions as a bidirectional voice frequency repeater, having automatic gain adjustment circuitry and auto-balancing circuitry for both equipment and cable side balance networks.
2. Description of the Prior Art:
As is generally known, it is often required to enhance signals which are being transmitted over a 2-wire bidirectional transmission path in telephone and other similar communication systems. In addition, it is often necessary to couple a 2-wire bidirectional transmission path to unidirectional sections of a 4-wire transmission path. For example, bidirectional signals in a telephone system are carried over the 2-wire suscriber loop but must be split into separate transmit and receive unidirectional signals at the central office and at the subscriber terminal. Signal enhancement in the bidirectional 2-wire transmission path is typically achieved by various forms of hybrid circuits to couple the 2-wire path to the separate unidirectional sections, each including unidirectional amplifiers.
Standard hybrids for analog voice frequency signals utilize multiwinding transformers arranged to couple signals from the transmit port to the 2-wire access port and to couple signals from the 2-wire access port to the receiving port. Further, it is desired that such hybrids also present to the 2-wire access port an impedance equal to the characteristic impedance of the connected 2-wire line in order to avoid the occurrence of a reflected signal and consequent echo. In addition, precision balancing networks have been used in these hybrid arrangements to match the impedance of the incoming lines.
However, it is known that the characteristic impedance of real 2-wire telephone lines is not purely a resistive impedance but has complex impedances with certain frequency-sensitive variations. As a result, it has been extremely difficult to adjust continuously the balancing networks so as to match the characteristic line impedance. Accordingly, most of the prior art bidirectional repeater circuits have suffered from the disadvantages of frequency instability and/or echo signals.
Examples of various prior art hybrid circuits, hybrid repeaters, and associated circuitry are shown and described in U.S. Pat. No. 4,331,842 to Kiko; U.S. Pat. No. 4,192,978 to Vincent; U.S. Pat. No. 4,181,824 to Seidel; U.S. Pat. No. 4,074,087 to Blake, Jr. et al.; U.S. Pat. No. 4,168,878 to Hashemi; U.S. Pat. No. 4,064,377 to Regan; U.S. Pat. No. 4,063,187 to Orchard; U.S. Pat. No. 4,174,470 to Seidel; U.S. Pat. No. 3,963,876 to Holtz et al.; and U.S. Pat. No. 3,689,711 to Earle et al.
It would be desirable to provide an enhanced line powered amplifier for effecting amplification in a 2-wire bidirectional transmission path which includes automatic gain adjustment circuitry for automatically setting the gain to a fixed level. It would also be expedient to provide an enhanced line powered amplifier which can automatically adjust the balance networks on both the equipment and cable sides. Further, the hybrid repeater is formed of a voltage sense amplifier, a shunt current driver and a shunt current compensator. Gain/ equalizer circuits are switchable between a loaded cable and a non-loaded cable.